ISL6529 Datasheet, PDF (12/19 Pages) Intersil Corporation – Dual Regulator.Synchronous Rectified Buck PWM and Linear Power Controller

ISL6529, ISL6529A
80
60
3A
40
55mA
20
1mA
0
BODE PLOT OF REGULATOR (N0 COMP) AT 3 OUTPUT CURRENTS
0
3A
-50
-100
55mA
-150
-200
10
1mA
1mA
55mA
100
1K
10K
100K
1M
10M
FREQUENCY (Hz)
FIGURE 10. LOOP RESPONSE WITH ONLY 100µF, 5mΩ
OUTPUT CAPACITOR
The Compensation Network
To provide for system stability with a low ESR output
capacitor, where pole cancellation by the capacitor is outside
the frequencies of interest, a phase lead network must be
used to compensate for the phase lag resulting from
MOSFET output resistance and the output capacitor.
INTERNAL 0.8V
REFERENCE
+
ISL6529
-
FB
DRIVE 2
TO MOSFET
REGULATED
OUTPUT
C4
R12
R5
C16
8800
----------------¹------------------
2π ⋅ R12 ⋅ C4
----------------------------1------------------------------
 ^{C16 ⋅ C4} 
6700
2π
⋅
R12
⋅


C-----1---6-----+-----C-----4--_
R12
4₆0₀
20 ⋅ log ----------
R5
2500
400
300
-4250
-9100
-1350
1100
110000
11,00K0
110,000K0
11000,0000K
FREQUENCY (Hz)
1,0100,M000
FIGURE 11. SCHEMATIC AND BODE PLOT OF
COMPENSATION NETWORK
110,0000,M000
Figure 11 shows the type II compensation network
configuration and a simplified straight line representation of
the network response. By using the equations in Figure 11,
the values of R12 and C4 can be adjusted to the frequency
where phase lead begins. The second equation shows the
upper frequency where phase advance is complete. After
capacitor C4 becomes effectively an ac short, the mid band
gain is set by the R12/R5 ratio as shown on Figure 11.
Several simulations illustrate the compensation with the
more difficult 5mΩ, 100µF output capacitor. Figure 10
previously showed the Bode plots where the phase
response comes dangerously near the oscillatory 180° state
at unity gain. A type II network applied across the error
amplifier inverting input to the output can be configured to
perform this function.
80
COMPENSATION:
60
C16 = 10pF
C4 = 470pF
40
R16 = 200K
20
1mA
0
3A
BODE PLOT OF REGULATOR (COMP ONLY) AT 3 OUTPUT CURRENTS
0
-50
-100
-150
-200
10
54mA
1mA
3A
100
1K
10K
100K
1M
10M
FREQUENCY (Hz)
FIGURE 12. LOOP RESPONSE WITH COMPENSATION
NETWORK ONLY
80
COMPENSATION:
60
C16 = 10pF
C4 = 470pF
40
R16 = 200K
20
1mA
3A
54mA
0
BODE PLOT OF COMPENSATED REGULATOR AT 3 OUTPUT CURRENTS
0
-50
-100
135°
-150
3A
54mA
1mA
-200
10
1mA
100
1K
3A
10K
100K
1M
10M
FREQUENCY (Hz)
FIGURE 13. INVERTING INPUT TO OUTPUT GAIN AND
PHASE WITH COMPENSATION AND 100µF, 5mΩ
OUTPUT CAPACITOR
Figure 12 shows the Bode plot of only the compensation
network with the system to illustrate the phase boost in the
system. Essentially the phase lead region must be moved to
advance the phase where it is close to an oscillatory state.
Increasing the value of C4 moves the response lower, aiding
the lagging phase at low frequencies and low load current.
This operation will reduce phase compensation at higher
12
FN9070.5
April 12, 2005